Unraveling the role of polymeric ferric sulfate in promoting propionic acid in anaerobic sludge hydrogen production fermentation

Polymerized ferric sulfate (PFS) is widely employed in the pretreatment of industrial wastewater and sludge dewatering. Its widespread use results in its accumulation in waste activated sludge (WAS) and may impact anaerobic fermentation (AF). This work fills the gap in the effects and mechanisms of PFS on hydrogen (H2) production from AF under mesophilic and thermophilic conditions. PFS (40 g/kg total solids, TS) reduced cumulative H2 production by 59.2% (mesophilic conditions) and 52.6% (thermophilic conditions) (p < 0.001), whereas propionic acid production increased by 299.2% and 192.0%, respectively. Model-based analysis showed that PFS inhibited solubilization, hydrolysis, and acidification below 10 g/kg TS but promoted them above 10 g/kg TS. Changes in the ratio of the two forms of nicotinamide adenine dinucleotide (NADH/NAD+) and hydrogenase activity led to unfavorable pathways for H2 production. Thermophilic fermentation exhibited strong tolerance to PFS due to the increased release of extracellular polymeric substances (EPSs). By analyzing 16S-rRNA gene operational taxonomic units (OTUs), the effects of environmental factors on microbial communities and microbial interactions under the combined effects of PFS and temperature were identified, revealing the important factors affecting H2 production. Sludge containing PFS was not conducive to anaerobic H2 production; however, mesophilic fermentation with PFS may be a potential way to recover propionic acid with low energy consumption. This study provided insights into the effects of potential contaminants in WAS on AF for H2 production and information for the effective disposal and resource utilization of WAS.